A flexible printed wiring board is produced by etching a copper foil of a substrate to form various wiring patterns and connecting electronic components with solder to mount the same. A copper foil is classified into an electrolytic copper foil and a rolled copper foil according to the methods for producing the same, and the rolled copper foil which is excellent in flexing resistance has been preferably used as a copper foil for a flexible substrate. Further, in electronic devices such as personal computers and mobile communications, a higher-frequency electric signal has been used with an increase in speed and capacity of communication, and a printed wiring board and a copper foil which can meet the trend have been required.
A higher-frequency electric signal has been used in electronic equipment such as personal computers and mobile communications, but when the frequency of an electric signal is increased to 1 GHz or greater, the influence of skin effect in which current flows only through the surface of a conductor becomes remarkable, and the influence in which a current transmission path is changed by surface unevenness to increase conductor loss cannot be ignored. Also from this point of view, the surface roughness of a copper foil is desired to be small.
The surface of a raw electrolytic copper foil is formed of electrodeposited particles of copper, and the surface of a raw rolled copper foil is formed by contact with a reduction roll. Therefore, the surface roughness of the raw rolled copper foil is generally smaller than the surface roughness of the electrolytic copper foil. Further, the rolled copper foil is finer than electrodeposited particles in roughening treatment. From this meaning, it can be said that a rolled copper foil is excellent as a copper foil for high frequency circuits.
On the other hand, data transportation volume is increased with the increase in frequency, but the circuit length of FPC is limited because the loss (damping) of signal power is also increased and the data cannot be read. In order to reduce such loss (damping) of signal power, the surface roughness of a copper foil tends to be smaller on the conductor side, and the resin side tends to be shifted from polyimide to a liquid crystal polymer. Note that a copper foil with small roughness in which a roughening treatment layer is not formed will probably be most desirable from the point of view of skin effect.
The loss (damping) of the signal power in an electronic circuit can roughly be divided into two. One of them is conductor loss, that is, the loss by a copper foil, and the other is dielectric loss, that is, the loss by a substrate. The conductor loss has characteristics that, in a high frequency area, there is a skin effect and current flows through the surface of the conductor. Therefore, when a copper foil surface is rough, current will flow through a complicated path. As described above, since a rolled copper foil has smaller roughness than an electrolytic copper foil, the rolled copper foil tends to have a smaller conductor loss.
On the other hand, a liquid crystal polymer (LCP) is a polymer which shows optical anisotropy in a liquid phase (melting or solution), and the polymer needs to be laminated to a copper foil without an adhesive. A wholly aromatic polyester-based liquid crystal polymer is a halogen-free material which shows molecular orientation even in a molten state, maintains the molecular orientation even in a solid state, and shows thermoplasticity.
The liquid crystal polymer (LCP) is characterized by a lower dielectric constant and a low dielectric tangent. Incidentally, LCP has a relative dielectric constant of 3.3, while polyimide has a relative dielectric constant of 3.5 and LCP has a dielectric dissipation factor of 0.002, while polyimide has a dielectric dissipation factor of 0.01. Thus, the liquid crystal polymer (LCP) has better characteristics than polyimide. Further, the liquid crystal polymer (LCP) is characterized by a low water absorption property and a low coefficient of moisture absorption, and has a large advantage of a small change in electrical properties and a small dimensional change.
The rolled copper foil is characterized in that a rolled material obtained by rolling after final annealing is the best for the purpose of maintaining handleability (for example, refer to Patent Literature 1).
However, the liquid crystal polymer (LCP) has a large problem of having low strength as compared with polyimide so that a material to which a copper foil is laminated hardly shows high peel strength. If roughness of a copper foil is increased, peel strength will tend to be high because a physical anchor effect is obtained, but electrical properties at high frequency will get worse due to the influence of the skin effect as described above.
Further, although there are several proposals of a copper foil for high frequency circuits (for example, refer to Patent Literatures 2, 3, 4, and 5), there is no effective technique under the present circumstances from the point of view of simplification of a process for producing a rolled copper foil and reducing high frequency transmission loss.